The long-term goal of this proposal is to determine if the success of in utero hematopoietic stem cell transplantation can be improved with co-transplantation of facilitating cells (mesenchymal stem cells and alloreactive natural killer cells). We hypothesize that the use of these cells will improve the competitiveness of donor stem cells and ultimately increase the level of chimerism that is achieved after in utero hematopoietic transplantation. Theoretically, in utero transplantation of hematopoietic stem cells should treat and cure many heritable disorders of the lymphohematopoietic system. Target diseases potentially include hemoglobinopathies, immune deficiencies, and leukodystophies. Most of these disorders exert their deleterious effects prior to birth making the rationale for in utero treatment imperative. The feasibility of in utero therapy is supported by a variety of natural occurring human and animal chimeras that demonstrate high levels of stable mixed chimerism. However, successes in both experimental models and in the human fetus have not been as encouraging as the levels of chimerism achieved would not be expected to successfully treat human fetal disease. Two major exceptions are the fetal-sheep model and settings where there is a competitive advantage for donor cells as for example in W/W mice and in fetuses with severe immune defects (e.g. X-SCID). While the fetus may theoretically be an ideal candidate for therapy it is clear that significant barriers exist that prevent clinically relevant levels of chimerism from being achieved. Transplantation methods that will improve the competitiveness of donor cells must be developed if the potential of fetal transplantation is going to become a clinical reality. In this application we will determine if engraftment and chimerism can be improved with the use of hematopoietic facilitating cells. We also will evaluate the ability of these facilitating cells to reduce the risk of in utero graft verses host disease. If our hypotheses are correct, then in utero treatment for hematopoietic and immune disorders should be both safer and successful. [unreadable] [unreadable]